The invention relates to a nonvolatile one-time-programmable memory cell.
Prior art nonvolatile memories, such as Johnson et al. U.S. Pat. No. 6,034,882, “Vertically Stacked Field Programmable Nonvolatile Memory and Method of Fabrication,” have been based on a memory cell pairing a semiconductor junction diode with a dielectric antifuse layer, the diode and the dielectric antifuse layer disposed between conductors.
When the memory cell is formed, the dielectric antifuse layer (typically a layer of silicon dioxide) acts as an insulator, and when a read voltage is applied between the conductors, very little current flows between the conductors. When a sufficiently large voltage is applied between the conductors, however, the dielectric antifuse layer suffers dielectric breakdown and ruptures, and a permanent conductive path is formed through the dielectric antifuse layer.
In a programmed cell, when a read voltage is applied between conductors, a significantly higher current flows than in the unprogrammed cell, allowing the unprogrammed and programmed cells to be distinguished. The memory state is stored in the state of the dielectric antifuse layer, which may be intact or ruptured.
Memory cells based on rupture of a dielectric antifuse layer, however, suffer some disadvantages. If the dielectric antifuse layer is too thin, leakage current can be a severe problem. Disturb can also be a problem: every time the memory cell is read, the dielectric antifuse layer is exposed to some stress, and may eventually break down and be inadvertently programmed. This is avoided by making the dielectric antifuse layer thicker, but a thicker dielectric antifuse layer requires higher programming voltage to rupture.
Higher voltages in electronic devices, for example in portable devices, are generally disadvantageous. If the dielectric antifuse layer is an oxide layer formed by oxidation, a thicker antifuse layer calls for either higher temperatures or slower fabrication time, both disadvantageous in forming a commercial device.
There is a need, therefore, for a one-time programmable memory cell which does not rely on rupture of a dielectric antifuse layer.